In a production line for packaging semiconductor chips (the term "semiconductor chip" or "chips" has been used interchangeably herein with the terms "semiconductor devices" and "semiconductor packages"), a wire bonding operation is used to create wire bonds between portions of semiconductor chips and selected portions of associated leadframe strips. The output of semiconductor chips after the wire bonding operation is inputted into a molding portion or step. The molding step encapsulates semiconductor chips in plastic. The encapsulated semiconductor chips and their associated leadframe strips leaving the molding step enter a post mold curing and cool down step.
Following curing and cooling of the encapsulated semiconductor chips, a dambar cutting and debris removal step removes dambars from the leadframe strips and dislodges excess debris from the plastic housing for each of the semiconductor chips. A plating step or operation plates selected portions of the leadframe strips with a selected plating material while an off-load/on-load step is between the dambar cutting and debris removal step and the plating step. The output from the plating step is inputted into a trim and form step or operation that both removes excess portions from the leadframe strips by cutting and shapes portions of the leadframe strips by bending extended leads thereof extending from the semiconductor chips. A marking step or operation typically follows in which the plastic housing for each semiconductor chip is marked with an identification designation. The marking step may alternatively be done before the plating step. The offload section of the trim and form/marking steps or operation delivers each of the assembled and completely packaged semiconductor chips to magazines. The above steps are each typically performed by stand-alone systems. A leadframe strip may be configured to include a single row of semiconductor chips or it may be configured as a multi-row (matrix) configured leadframe strip. A process area on a leadframe strip includes one or more semiconductor chips depending on leadframe strip configuration.
Conventional processing has included a linear track along which has been positioned one press having a top press plate and a bottom press table, between which is a tool house for each tool performing a step of the process. As the leadframe strip travels along the track, the top press plate typically moves vertically opening and closing the press. The tools have disadvantageously been operated simultaneously i.e. one could not close one tool at a time because top press plate movement affects all the tools. Previously, all the packages (plastic encapsulated semiconductor chips) on one leadframe strip had to go through a first tool by moving forward one process pitch at a time before reaching a second tool. Leadframe strip transport and press movement have typically been controlled by software sequences one movement at a time.
In order to position each leadframe strip correctly within each station performing the various steps, each leadframe strip contains alignment holes that match corresponding alignment pins in the supporting equipment, with which they become engaged for proper positioning at each portion. These same alignment holes are used as the leadframe strip progresses along the track for processing. Since each step is carried out within very narrow tolerances, it is extremely important that these alignment holes, particularly the center thereof, remain in constant spatial relationship with respect to the rest of the leadframe strip. Moreover, it is important that the alignment holes are not damaged during leadframe strip transport or during positioning inside a tool. Unfortunately, the plastic molding step subjects the leadframe strip to structural and thermal stresses caused by differences in the heat expansion coefficients of the metal and the plastic material. Thus, during the cooling phase, the plastic encapsulated leadframe strip is stressed by the cooling plastic and is subjected to bending, waving and twisting of the leads as well as shrinkage causing leadframe strip pitch changes and warping or tensioning of the leadframe strips. This deformation often results in a misalignment of the alignment holes in the leadframe strip with the receiving pins in successive workstations, which causes a mismatch between the holes and the pins causing considerable quality control problems.
Because of the foregoing structural and thermal stresses, the tendency toward more precision devices with smaller dimensions, more exacting tolerances and a matrix configured leadframe strip has led to the conclusion that a leadframe strip must be processed progressively instead of at once. Moreover, each leadframe strip can be processed by only one tool at a time to prevent positioning of the leadframe strip by more than one tool.
Therefore, tools in one press should have been positioned a minimum of about 250 mm apart from each other (assuming a maximum leadframe strip length of 250 mm). However, tool accuracy worsens and production costs increase the longer the distance from tool to tool. Moreover, the amount of production floor space required by these arrangements is extensive, particularly when necessary to process a number of leadframe strips at the same time.
Accordingly, there has been a need for a novel tool drive apparatus and method to accurately process more than one process area on a leadframe strip in the same press cycle. There is an additional need for a tool drive apparatus and method that enable tools to be placed closer together than the length of a leadframe strip so as to shorten the processing line taking up only a fraction of the previously-used production floor space to lower costs and to increase accuracy and efficiency. A still further need exists for a tool drive apparatus and method that offers more processing flexibility, higher throughput, and that can be quickly and inexpensively retooled to perform different processing steps at each station to process different types of leadframe strips as well as process different semiconductor packages. There is a still further need for a tool drive apparatus and method to be used with a drive unit that operates more than one press and that only uses the force to drive the tool that has the heaviest process operation. Additionally, there is a need for a tool drive apparatus and method in which the leadframe strip can be positioned in one tool at a time. The present invention fulfills these needs and provides other related advantages.